Electrochemical Methods for Nutrient Removal in Wastewater: A Review of Advanced Electrode Materials, Processes, and Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

See the attached file, please.

Comments for author File: Comments.pdf

Author Response

Comments 1: The scheme of chemical reactions on P. 14, equations 18, 19, is clearly wrong. It mixes up Fe(II) and Fe(III). Fe(OH)2 cannot form from Fe3+ without a reducing agent. The equation (19) is out charge balance.

Response 1: Thank you for your feedback. I have revised the equations to correct the chemical reactions involving iron electrodes. Previously, Fe(OH)₂ was incorrectly shown to form from Fe³⁺, which is not possible without a reducing agent. The updated equations now show the correct sequence: Fe²⁺ is formed at the anode (Eq. 17), which then reacts with water to form Fe(OH)₂(Eq. 18). Finally, Fe(OH)₂ is oxidized in the presence of oxygen to form Fe(OH)₃, resolving the issue of charge balance. Additionally, a new equation has been added to account for this oxidation step, and the equation numbering has been updated to reflect these changes, now ranging from (15) to (17).

Comments 2: The equation (11) also raises questions. The authors wrote that the standard electrode potential of the Fe(IV)/Fe(III) pair is ca. 2.0 V (SHE). On the other hand, the standard electrode potential of hydrogen peroxide reduction H2O2 + 2 H+ 2e ⇄ 2 H2O is 1.776 B (SHE). Therefore, hydrogen peroxide cannot oxidize iron to +4 oxidation state.

Response 2: Thank you for your valuable comment. Based on your feedback, I have removed the previous reaction (Eq. 11) and the related discussion on Fe(IV), as it was not thermodynamically feasible. The revised text now focuses on other reactive oxygen species (ROS), such as superoxide anion (O₂•⁻), which are more relevant to the electrocoagulation process. This revision ensures the scientific accuracy and relevance of the discussion.(p10.line1-3)

Comments 3: P.6, line 39. Data on the chemical oxygen demand reduction are meaningless without specifying the pollutant, its initial concentration, current density, geometry of the electrochemical cell, etc. Antibiotics include a wide range of substances; the oxidation rate of different antibiotics can vary.

Response 3: Thank you for your valuable comment. In response to your feedback, I have added specific details regarding the experimental conditions relevant to the COD reduction data. The revised text now specifies the pollutant (ampicillin), its initial concentration (105 mg/L), and the current density (50 mA/cm²) used in the study. Additionally, I have acknowledged that the oxidation rate can vary between different antibiotics due to their molecular structure and functional groups, which may result in varying efficiency for other types of antibiotics. This ensures that the data is meaningful and accurately reflects the experimental setup.(p6.line35-44)

Comments 4: The issue of using aluminum anodes for electrocoagulation should be considered in much more detail. The fact is that aluminum is easily passivated under anodic polarization conditions. Passivation of aluminum anodes results in higher energy consumption and lessens the effectiveness of electrocoagulation process. It is necessary to clarify the composition of the solution.

Response 4: Thank you very much for your valuable feedback. In light of your suggestion, I have simplified the discussion surrounding the use of aluminum anodes in electrocoagulation. Given the well-documented challenges of aluminum anode passivation, which can lead to increased energy consumption and reduced process efficiency, I felt that minimizing the focus on aluminum would help to maintain clarity and avoid confusion. Furthermore, to ensure the discussion remains focused and relevant, I have removed equations (16, 17), which were related to aluminum reactions but not critical to the main arguments presented.

Comments 5: Pt/Ti and Pt/Nb are also widely used for the electrochemical treatment of wastewater. They should be included in Table 2.

Response 5: Thank you for the valuable feedback. Based on your suggestion, Pt/Ti and Pt/Nb electrodes have now been included in Table 2 as they are indeed widely utilized for the electrochemical treatment of wastewater.

Comments 6: The change in pH during electrochemical treatment is very important. Electrooxidation of an ammonia molecule leads to the formation of nine hydrogen ions per eight electrons (equation 14), whereas the reduction of water molecules requires two electrons and results in the formation of two hydroxyl ions (the coefficients in equation 15 can be reduced). Therefore, the overall equation for the reaction in the electrolyzer will be NH3 + 3 H2O → NO3– + H+ + 4 H2. A neutral or slightly alkaline solution will be acidified during the electrolysis process. The authors should discuss this issue briefly.

Response 6: Thank you for your valuable comment. Upon reviewing the original equations, I have identified that the previous representation of the electrochemical reactions was incorrect. Based on your feedback, I have revised both the equations and the corresponding explanations to more accurately reflect the ammonia oxidation process. The changes now correctly describe the oxidation of ammonia and the overall reactions occurring in the electrolyzer, including the acidification of the solution during electrolysis. The revised equations now better represent the dynamics of the reaction and address the pH changes during the process, resolving the inaccuracies present in the previous version.(p11.line12-p12.line8)

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript describes the recent technologies and advances in the electrochemical methods. This review concludes several technologies that involve the N and p removal by several eletrodes. Importantly, this review also presents the cost limitation and burdening factors of the technologies. However, the manuscript quality needs to be elevated in order to meet the acceptance standard. Several notes that need to be addressed are as follow:

1. Fig 1, needs to be enriched with more data from other databases. It helps the readers to comprehend more about the growth of documents concerning this technology. Furthermore, more detail regarding this data retrieving also important and needs to be mentioned.

2. Table 2. Electrode materials need to be mentioned in the full name instead of abbreviation. It is also important to put details of capacity, efficiency, and other application details based on the references.

3. Table 5 is too long and too much information to add at the same time. Please clarify and improve the details.

4. Practical application needs present the actual representatives of each technologies. It is important to discuss more about this. Moreover, please make a cost efficiency in the same unit so the comparison can be made accordingly.

5. The discussion regarding the challenge with other technologies to remove nutrient needs to be compared as well. The technology may stand out in terms of electrochemical process but not if it is compared to other method of nutrient removal.

6. Wastewater treatment is a complicated process. Please elaborate the possibility of coupling this technology with certain process during the treatment proces.

7. future prospect of this technology and direction where the studies needs to be conducted on also important to mention. Based on the flaws that mentioned it is important to have it discussed in this aspect as well.

Author Response

Comments 1: Fig 1, needs to be enriched with more data from other databases. It helps the readers to comprehend more about the growth of documents concerning this technology. Furthermore, more detail regarding this data retrieving also important and needs to be mentioned.

Response 1: Thank you for your valuable feedback. To enrich Figure 1, I have incorporated additional data from the Web of Science database, providing a broader view of the growth of documents concerning this technology. The same keywords ('electrochemistry', 'water', 'electrode', 'treatment', 'wastewater') were used for searching documents from 2014 to 2023, and the results were categorized into experimental studies and review articles(p4.line24-30). Furthermore, I have added detailed explanations regarding the data retrieval process. The keywords, time frame, and criteria used for distinguishing between experimental and review articles have been clearly outlined to help readers better understand how the data was collected and categorized.

Comments 2: Table 2. Electrode materials need to be mentioned in the full name instead of abbreviation. It is also important to put details of capacity, efficiency, and other application details based on the references.

Response 2: Thank you for your valuable feedback. In response, I have updated Table 2 by replacing all abbreviations with the full names of the electrode materials. Additionally, I have included detailed information on capacity, efficiency (such as COD/TOC removal rates), and application areas for each electrode, particularly in industrial and wastewater treatment contexts, based on the references. These updates provide a clearer and more comprehensive comparison of the electrodes' performance and potential applications.

Comments 3: Table 5 is too long and too much information to add at the same time. Please clarify and improve the details.

Response 3: Thank you for the insightful comment. In response to your suggestion, Table 5 has been revised to improve clarity and conciseness. The key methodologies and results are now summarized, focusing on the most important aspects of each approach.

Comments 4: Practical application needs present the actual representatives of each technologies. It is important to discuss more about this. Moreover, please make a cost efficiency in the same unit so the comparison can be made accordingly.

Response 4: Thank you for your insightful comment. I agree that discussing practical applications and presenting actual representatives of each technology is important. However, as this research focuses predominantly on pilot and lab-scale studies, there are currently limited examples of large-scale industrial implementations of these technologies. Most of the available data comes from controlled environments rather than full-scale industrial settings. To address this limitation, future research should aim to conduct full-scale pilot tests or industrial applications that can provide more concrete data on the performance and cost efficiency of these technologies in real-world scenarios. This will help bridge the gap between lab-scale experiments and practical industrial applications.

And in response to your comment, I have standardized the units in Table 8 to kWh/m³ to allow for clearer comparison across the different technologies.

Comments 5: The discussion regarding the challenge with other technologies to remove nutrient needs to be compared as well. The technology may stand out in terms of electrochemical process but not if it is compared to other method of nutrient removal.

Response 5: Thank you for your insightful comments. In response to your suggestion, I have incorporated a comparative analysis of electrochemical methods versus other nutrient removal technologies in terms of cost, scalability, and nutrient removal efficiency. This comparison is now presented in Table 6(p14.line46-p15.line5), which highlights the advantages and challenges of electrochemical methods when compared to traditional treatment technologies. I believe this addition provides a clearer context for evaluating the overall performance of electrochemical processes within the broader landscape of wastewater treatment technologies.

Comments 6: Wastewater treatment is a complicated process. Please elaborate the possibility of coupling this technology with certain process during the treatment process.

Response 6:. Wastewater treatment is indeed a complex process, and coupling electrochemical (EC) technologies with other treatment methods can significantly enhance overall efficiency. Specifically, combining EC processes with biological treatments, such as activated sludge or anaerobic digestion, can improve the degradation of complex organic pollutants. This integration allows biological processes to handle biodegradable matter while EC technologies focus on removing nutrients and other persistent contaminants. This combined approach offers a more robust and flexible solution for wastewater treatment, optimizing both performance and sustainability, as seen in studies such as those highlighted by Yakamercan et al. (2023) in their comprehensive review of electrochemical treatment systems combined with biological processes for wastewater remediation (Yakamercan et al., 2023, https://www.sciencedirect.com/science/article/pii/S0269749123006826).

Comments 7: future prospect of this technology and direction where the studies need to be conducted on also important to mention. Based on the flaws that mentioned it is important to have it discussed in this aspect as well.

Response 7: Thank you for your valuable comment. In response to your suggestion, I have expanded the discussion on the future prospects and research directions of EC technologies in the conclusion section. Specifically, I have emphasized the potential of hybrid systems that combine EC processes with complementary technologies, such as biological treatments and membrane filtration, to improve overall efficiency and broaden their industrial applications. These hybrid approaches offer significant benefits by addressing the limitations of standalone EC systems and optimizing performance in terms of both cost-effectiveness and energy efficiency. Additionally, I have elaborated on the need for further advancements in automation and real-time monitoring to help maintain optimal conditions, reduce electrode fouling, and minimize operational costs in large-scale applications.(p18.line23-40)

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript “Novel Electrodes for Nutrient Treatment in Wastewater Using Electrochemical Methods: A Review of Processes, Applications and Challenges has been reviewed. The review examines technologies for the electrochemical treatment of wastewater, focusing on nitrogen and phosphorus in wastewater. Electrochemical treatment methods such as electrocoagulation, electroflocculation, electrodeionization, and electrochemical oxidation were highlighted and discussed. Electrochemical treatment methods for wastewater are promising and green. It reduces the problems of secondary pollution, chemical consumption and scalability. The manuscript would be a good addition to the body of knowledge and is suitable for publication. However, the following updates are necessary before acceptance and publication.

1.      The abstract section should be strengthened about the novelty of the study

2.      A table showing a comparative analysis of electrochemical methods in terms of cost and scalability etc over other treatment technologies should be incorporated into the manuscript

Author Response

Comments 1: The abstract section should be strengthened about the novelty of the study

Response 1: The abstract has been revised to emphasize the novelty of the research and its industrial applicability.

Comments 2: A table showing a comparative analysis of electrochemical methods in terms of cost and scalability etc over other treatment technologies should be incorporated into the manuscript

Response 2: Thank you for your insightful comments. In response to your suggestion, we have incorporated a comparative analysis of electrochemical methods versus other nutrient removal technologies in terms of cost, scalability, and nutrient removal efficiency. This comparison is now presented in Table 6(p14.line46-p15.line5), which highlights the advantages and challenges of electrochemical methods when compared to traditional treatment technologies. We believe this addition provides a clearer context for evaluating the overall performance of electrochemical processes within the broader landscape of wastewater treatment technologies.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

The title of the manuscript starts with novel electrodes which gives the impression that the manuscript explores newer electrodes developed by the group. In fact, it is not like that. Better to change the title and provide a suitable one related to the review. 

The abbreviations of the electrodes, materials and processes should be expanded on the first time. This is not followed throughout the manuscript. A thorough check is needed. 

Page 2, Line 35: "such as N and which play" something is missing here. 

Table 1: Check electrochemical oxidation: repeated statements in column 3. 

Check the captions of Tables 2 and 3. One of them is wrongly attributed. Also, the expansions given can be directly given in the table itself. For example, Activated carbon (AC), like this. 

Page 10, Line 17: Nitrogen as ammonia, but the ion formula given is for ammonium. Which is correct?

Page 12, equation 14: the description before it is wrong. 

Page 13, Line 16: Ehen, what does it mean?

The same idea is repeated in several places. For example, general description of electrocoagulation is given in introduction as well as phosphate removal. Proper check is required to remove repetition. 

Table 6: Caption not appropriate. What is the need of this table? The contents can be described in the section itself. 

The reaction with phosphates and nitrogen are not provided. Only the reaction of electrodes are provided. The reaction with the species will give an idea to reader what exactly is happening. 

MBR is not expanded anywhere. 

Figure 2 is not mentioned anywhere in the running text. 

Page 16, Line 4: w missing from well. 

Proper review of the written material is recommended by all authors to have a well written review of the subject. Authors have reported a large number of references; however, their proper review is missing. More efforts needed to have a comprehensive review. 

Comments on the Quality of English Language

Some sentences have typographical errors and grammar mistakes. 

Author Response

Comments 1: The title of the manuscript starts with novel electrodes which gives the impression that the manuscript explores newer electrodes developed by the group. In fact, it is not like that. Better to change the title and provide a suitable one related to the review. 

Response 1: Thank you for the suggestion. We agree that the current title may imply a focus on newly developed electrodes by our group, which is not the case. We have revised the title to better reflect the manuscript's scope as a review of electrochemical methods for nutrient removal, with a focus on advanced electrode materials, processes, and applications. The new title is "Electrochemical Methods for Nutrient Removal in Wastewater: A Review of Advanced Electrode Materials, Processes, and Applications."

Comments 2: The abbreviations of the electrodes, materials and processes should be expanded on the first time. This is not followed throughout the manuscript. A thorough check is needed. 

Response 2: I have reviewed the manuscript to maintain consistency in abbreviation usage, ensuring that all terms are properly introduced in their expanded form before using abbreviations. I appreciate your attention to detail, and we believe this revision enhances the readability and clarity of the manuscript.

Comments 3: Page 2, Line 35: "such as N and which play" something is missing here. 

Response 3: I have revised the sentence for clarity. The corrected sentence now reads: "Electrochemical processes are gaining attention as effective methods for the removal of key nutrients, such as nitrogen (N) and phosphorus (P), which play a critical role in water quality and protecting ecosystems by effectively targeting nutrient pollution." This revision ensures that the sentence flows naturally and accurately conveys the importance of nitrogen and phosphorus in the context of nutrient pollution.(p2.line 35-37)

Comments 4: Table 1: Check electrochemical oxidation: repeated statements in column 3. 

Response 4: Thank you for your observation. I have revised the Electrochemical Oxidation section in Table 1 to eliminate the repeated statements in the Advantage column.

Comments 5: Check the captions of Tables 2 and 3. One of them is wrongly attributed. Also, the expansions given can be directly given in the table itself. For example, Activated carbon (AC), like this. 

Response 5: I've updated the tables to include expanded abbreviations directly within the table for improved clarity. For instance, abbreviations are now provided in parentheses within the tables.

Comments 6: Page 10, Line 17: Nitrogen as ammonia, but the ion formula given is for ammonium. Which is 2. correct?

Response 6: After reviewing the reaction mechanisms, I have clarified that the correct form in this context should be ammonium (NH₄⁺), as this is the predominant species in aqueous solutions under the conditions described. The relevant chemical equations have been updated to reflect this distinction. (p11.line12-p12.line4)

Comments 7: Page 12, equation 14: the description before it is wrong. 

Response 7: I have corrected the description and equation (14) to accurately reflect the appropriate reaction .(p11.line12-p12.line4)

Comments 8: Page 13, Line 16: Ehen, what does it mean?

Response 8: Thank you for catching that typo. I have corrected "Ehen" to "When" in the sentence.

Comments 9: The same idea is repeated in several places. For example, general description of electrocoagulation is given in introduction as well as phosphate removal. Proper check is required to remove repetition. 

Response 9: Thank you for your feedback regarding the overlap between the Introduction and Electrocoagulation for Phosphate Removal sections. I have revised the manuscript to eliminate redundant information. In the Introduction, I now focus on the general advantages of electrochemical methods and the importance of phosphorus removal. The Electrocoagulation for Phosphate Removal section, on the other hand, has been refined to provide specific details on electrocoagulation, including its mechanisms, applications, and effectiveness in phosphate removal. This revision ensures that each section contributes unique information and improves the overall clarity and readability of the manuscript.

Comments 10: Table 6: Caption not appropriate. What is the need of this table? The contents can be described in the section itself. 

Response 10: Thank you for the suggestion. We agree that Table 6 can be effectively integrated into the main text for a more seamless flow. We have removed the table and expanded the discussion within the "Electrocoagulation for Phosphate Removal" section to include relevant details from recent studies, including specific examples and research findings.

Comments 11: The reaction with phosphates and nitrogen are not provided. Only the reaction of electrodes is provided. The reaction with the species will give an idea to reader what exactly is happening. 

Response 11: Thank you for your valuable comment. The reactions involving phosphates and nitrogen were intentionally omitted to avoid overwhelming the reader with too much complexity. Our goal was to focus on the electrode reactions to maintain clarity and ensure the main concepts were accessible.

Comments 12: MBR is not expanded anywhere. 

Response 12: I have updated the manuscript to expand the abbreviation "MBR" to Membrane Bioreactor at its first mention.(p14.line 43)

Comments 13: Figure 2 is not mentioned anywhere in the running text. 

Response 13: Thank you for pointing this out. We have added a sentence to the relevant paragraph to explicitly reference Figure 2. The new sentence reads: "Figure 2 illustrates a structural schematic of the electrolytic cells used in these electrochemical processes. (p16. line31-32)

Comments 14: Page 16, Line 4: w missing from well. 

Response 14: I've identified and fixed the issue.

Thank you for your valuable feedback. We acknowledge the importance of thoroughly reviewing and integrating the references to provide a more comprehensive analysis of the topic. We are currently revising the manuscript to ensure that the references are critically evaluated and discussed in greater depth. This will strengthen the review’s coverage and contribute to a well-rounded perspective on electrochemical methods for nutrient removal. We appreciate your suggestion and are committed to enhancing the manuscript’s quality through a more thorough examination of the cited studies.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised version of the manuscript can be accepted.

Author Response

I hope this message finds you well. I have carefully addressed the comments from the first review, and I understand that my responses were noted with the feedback, "The revised version of the manuscript can be accepted." However, I am currently unable to verify in the system whether any additional comments from a second review have been provided.

If there are further comments from the second review, I would be grateful if you could share them. Otherwise, I would like to confirm that I have no additional comments or revisions to submit at this time. Please note that all other reviewers' feedback has already been addressed as requested.

Thank you very much for your assistance and for keeping me updated.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made significant changes to the manuscript. However, several notes still need to be considered for the further improvement, including:

1. Figure 1 needs to contains several databases. It is important to put all together to have more perspective. Does the number mean annual publication or does it mean cumulative number?

2. 3.2 title shall change since N is not only ammonia/ammonium.

3. Table 6. The term low, moderate, to high shall be specified.

Author Response

Reviewer 2

Comments 1. Figure 1 needs to contains several databases. It is important to put all together to have more perspective. Does the number mean annual publication or does it mean cumulative number?

Response 1: Thank you for your insightful comment regarding Figure 1. I appreciate your suggestion to include data from multiple databases to provide a broader perspective. Currently, Figure 1 presents data exclusively from the Scopus database, showcasing the annual publication trends from 2014 to 2023 for studies on electrochemical wastewater treatment. I classified the documents into experimental and review articles by analyzing the abstracts and categorizing each study as either reporting new experimental data or summarizing existing literature(p4. Line 24-26).

To clarify, the numbers in Figure 1 represent the annual publication count, not a cumulative total. Each bar corresponds to the number of publications added each year, reflecting the growing interest in electrochemical methods for wastewater treatment. I have also clarified this point in the manuscript to ensure it is evident to readers.

Thank you once again for this suggestion, as it aligns with my goal of presenting a thorough and accurate portrayal of research trends in this area.

Comments 2. 3.2 title shall change since N is not only ammonia/ammonium.

Response 2: Thank you for your comment regarding the title of section 3.2. I have revised the title to:

"3.2. Advanced Electrochemical Approaches for Nitrogenous and Phosphorus Compounds Removal"

This updated title reflects the broader range of nitrogen compounds discussed. I appreciate your suggestion to enhance the accuracy of the manuscript.

Comments 3. Table 6. The term low, moderate, to high shall be specified.

Response 3: Thank you for your comment regarding the terms "low," "moderate," and "high" in the cost parameter of Table 6. I have updated the table to define these terms more specifically: 'low' cost is less than $1/m³, 'moderate' ranges from $1 to $5/m³, and 'high' exceeds $5/m³ (p15.line 6-8). This adjustment clarifies the cost distinctions among the nutrient removal technologies compared in Table 6.

Thank you for helping improve the accuracy of this comparison.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have considred all the comments and modified the manuscript accordingly. Currently it can be accepted for publication in sustainability without any modifications. 

Comments on the Quality of English Language

A thorough check on the English language is needed before the final acceptance. 

Author Response

Reviewer 4

Comments: A thorough check on the English language is needed before the final acceptance.

Response:  Thank you for your detailed review and feedback on the manuscript. In response to your suggestion regarding language quality, I have carefully reviewed the text and made minor adjustments to improve grammar and word choice across various sections. These changes were aimed at enhancing clarity and readability without altering the scientific content or intent. Thank you once again for helping me refine this work.

Author Response File: Author Response.docx